Current global production of ethylene oxide (EO) exceeds 14 million tons per year. The current industrial process, direct gas phase production of EO, is expensive with 20-25% of the main reactant, ethylene, being combusted in an unselective reaction on the base catalyst, Ag. Several patents claim that promoters on Ag catalysts, such as Re, Cs, and co-fed organic chlorides, yield EO selectivities over 80%. However, only the effects of Cs and organic chlorides have been studied in detail. In order to establish a more fundamental understanding of the Ag and Re-Ag catalysts, traditional and high-throughput reactor studies were executed. As Fig. 1 shows, the Ag catalyst activity was dramatically affected by the calcination conditions, without change to the selectivity. Based on this result, it is thought that catalyst sintering decreases the activity with larger calcination times. Results from studies with Re-Ag indicate that Re increases the EO selectivity at the expense of catalyst activity. The optimum performance of 45% EO selectivity for the Re-Ag, compared to 30% for Ag, was found for catalysts with 25 ppm Re. Characterization, using SEM, shows that the addition of Re also causes an increase in the Ag particle size from 15-35 mm.  An analysis of the oxygen reaction orders showed that the Re-Ag catalyst presents a more uniform distribution of sites for oxygen adsorption than unpromoted catalysts.  Thus, it is hypothesized that Re is blocking/destroying Ag step sites, increasing the EO selectivity.

Fig. 1.  Performance of Ag catalysts calcined at 400^oC for 1-21 hours